1. Filed of the Invention
This invention relates to a process for the production of a gelatin-silver halide photographic emulsion, particualrly, it is also concerned with a novel flocculating agent used during the production thereof and a method of flocculating gelatin and a silver halide.
2. Description of the Prior Art
A gelatin-silver halide photographic emulsion is ordinarily prepared by (1) forming and growing fine silver halide crystals by the double decomposition of a silver salt and halide salt in the presence of gelatin, (2) controlling the silver ion concentration, hydrogen ion concentration, etc., in the system by removing excess halide salt and a salt formed by the decomposition, and, if necessary, (3) adding a suitable sensitizer, followed by ageing.
In processes for the production of emulsions, as have been long practiced, the step (2) of removing salts is carried out by cooling and gelling the emulsion, finely dividing the resulting gel and washing with water after the step (1) of forming the fine crystals. This process, however, has the disadvantage that not only is a large amount of water and a long time required for water washing, also a gelatin concentration of some extent is required for gelling, and it is thus necessary to thicken the reaction solution in the case of preparing an emulsion containing a small amount of gelatin for a certain amount of a silver halide, and it is often impossible to obtain desirable photographic properties.
In order to overcome the above described disadvantages, it has been proposed to prepare emulsions by flocculating silver halide fine crystals formed in the presence of a relatively small amount of gelatin together with gelatin, followed by washing and redispersing. For example, there are, as methods of effecting this flocculation, a method comprising using a large amount of an inorganic salt, a method comprising using an organic solvent and a method comprising using a gelatin derivative or synthetic high molecular substance. The method comprising using an inorganic salt requires a large amount of the inorganic salt, while the method comprising using an organic solvent has the disadvantage that it is difficult to recover the expensive organic solvent. The method comprising using a gelatin derivative or high molecular substance can be carried out effectively at relatively low cost, and, in the case of using a synthetic high molecular substance, it is added to a silver halide emulsion containing gelatin and then a complex of a gelatin-synthetic high molecular substance containing the silver halide fine crystals is flocculated by lowering the pH of the system or adding a polyvalent metal salt thereto. The thus flocculated substance is washed with water, redispersed by raising the pH and then applied to the intended use, optionally with the addition of gelatin or sensitizers, ageing, etc.
It has hitherto been proposed to use a copolymer of maleic acid as a flocculating agent for a photographic emulsion as a high molecular substance (see British patent specification No. 648,472, U.S. Pat. No. 2,772,165 and West German Pat. No. 1,145,485).
As a flocculating agent for photographic emulsions, compounds capable of achieving this purpose at smaller amounts thereof are advantageous. That is to say, the smaller the amount of it such an agent, the more excellent is the redispersibility and the less troubles are encountered in succesively effecting chemical ageing, spectral sensitization, development and color development.
Generally, flocculating is carried out while lowering the pH, but the pH of a suitable flocculation depends upon the kind of flocculating agent used. A flocculating agent that is not effective unless the pH is lowered at the time of flocculating often has a harmful influence on the photographic properties of silver halide particles in an emulsion, depending upon the pH. Therefore, it is desirable that the pH not be so lowered at the time of flocculating, if possible. In the foregoing patent, it is disclosed that the pH ranges from 4 to 5.5, but insufficient flocculating occurs unless the pH is lowered to 3 to 4 in the case of a relatively low concentration gelatin emulsion.
From an economical point of view, it is desirable to obtain a flocculating agent as cheaply as possible. In this aspect also, the prior art flocculating agents are insufficient.